Some of you have recently sent me notes asking for clarification between Bulldog's Jetting Primer and Mucker's Principles of Carburetion, with respect to what sounds like different explanations of how the Pilot circuit works in the Road Star's stock Mikuni carburetor.

A careful reading, and a little more understanding about the differences between how gasses flow (air) and liquids flow (gasoline) will help to clear this up.

After a thorough examination of one of my extra carbs, and an extra carb body (all components have been removed), here is what I am seeing. If you want to skip this article altogether, the net answer is that the PMS screw meters both the air (emulsion for fuel only) and the fuel flowing through the pilot circuit, at idle only. For our purposes in adjusting the circuit, it cuts off fuel delivery during idle. During normal and transitional operation (as the butterfly valve opens) it only controls fuel flow through one of four pilot circuit fuel delivery holes in the carburetor's throat.

The Road Star Pilot circuit has the following elements:
  • An air bleed jet in the front of the carb, just to the left and a little lower than the air bleed for the needle circuit which is dead center bottom of the carb's bell.
    • Provides primary air for emulsification, plumbed to the middle of the pilot jet's tower in the float bowl
    • Mixes air into the pilot circuit before delivery to the two downstream circuits Idle operation and normal operation)
  • The pilot Jet, in the top (bottom actually) of the pilot tower in the float bowl
  • One small hole in the throat of the carb, on the manifold side of the butterfly valve for fuel delivery.
  • The PMS screw, which controls flow though the small hole on the manifold side of the butterfly valve
  • two small holes directly under the butterfly valve for fuel delivery, positioned so that the butterfly valve closes them when it is completely closed.
  • One small hole on the slide side of the butterfly valve that provides additional air for the circuit when the throttle is completely closed, or additional fuel during normal operation

During normal engine operation (non idle) the velocity through the carb creates vacuum through all four of the small holes in the carb throat, three of which are not regulated by the PMS screw, through which the carburetor can use to provide fuel. Vacuum per hole is small as it is created by the velocity of the air going through the carb only. It is in fact such a small portion of the fuel the carb provides that we tend to think of it as non operational (or of little to no consequence) for normal operation where the needle circuit is active and providing most of the fuel.

During idle operation. and situations where the butterfly valve is nearly closed, the primary vacuum is coming from the engine itself, which has no way to get sufficient air to satisfy its air flow needs and tends to create a higher than normal vacuum as it attempts to draw more air than it can get with the butterfly valve closed.

As the butterfly valve closes, it closes off the two small pilot holes that are directly beneath it. This forces all the vacuum demand (or very nearly all) of the engine to the small pilot hole on the manifold side of the butterfly valve. This hole is considerably smaller than the four will be together, and the mix properties will change as the velocity and demand through this single hole will be considerably higher. Gasoline ( a liquid -not compressible or expandable) will come through in full strength, air ( a gas -both compressible and expandable) will tend to stretch (become less dense) to fill the vacuum difference and the resultant mix will be richer. The net effect would be a stream of gasoline rather than a mist or properly emulsified mix.

To correct this tendency to jet a stream of fuel rather than an emulsified mix, the closing of the butterfly valve activates the small hole in between the butterfly valve and the slide as an additional air bleed. The vacuum coming through the single hole on the manifold side of the butterfly valve creates demand (or vacuum) in the hole on the slide side of the butterfly valve too, which now reverses its flow and supplies the needed air for the pilot circuit to balance its flow and emulsification back to something closer to what is needed for normal idle operation.

Now back to the PMS screw. It is located at the end of a tube that comes from the chamber where this mixing of fuel from the pilot jet (actually fuel and air from the air bleed in the front of the carb - which are combined in the pilot tower in the float bowl) and the small front hole in the carb throat between the butterfly valve and the slide. As you turn the screw in, it closes the hole on the manifold side of the butterfly valve off, effectively closing the entire circuit during idle operation when the butterfly valve is closed, including the air being provided by the small hole that is in between the butterfly valve and the slide (by reducing the vacuum that causes it to draw air).

As you open the butterfly valve, the two holes directly beneath the butterfly valve open, and begin to supply fuel to the carb (in response to both engine vacuum and air velocity - which creates vacuum) with the hole located closest to the slide still providing air. The hole closest to the slide will continue to provide air until the difference in vacuum between it and the other small holes is equalized, at which point it becomes another source of fuel for the carburetor to supplement the other three small pilot holes.

The net of all this is that we have a very well thought out, and progressive pilot circuit. One that can continue to provide well emulsified fuel to the carburetor under a variety of conditions. The PMS screw meters fuel (for our purposes- actually an emulsified air fuel mix) provided to the engine during idle operation, and supplemental fuel for normal driving. For normal driving purposes, it only meters flow through one of the four small pilot fuel holes in the carb throat. The reminder are free flowing with no regulation other than how the butterfly valve changes the circuit for idle, as outlined above.

I know, way too much information, but someone besides me will want to know how this circuit works